The invention pertains to machine vision and, more particularly, to machine vision methods for determining spatial characteristics of three-dimensional objects, such as the solder bumps on ball grid array devices and "flip" chips.
In automated assembly processes, such as those for assembling electronic circuit boards, it is important to verify the existence and robustness of components prior to placing them for assembly. For example, in order to make assemblies with ball grid array (BGA) devices and flip chips, an automated assembly system must verify the existence of solder "bumps" on those devices and chips, before attempting to align them for soldering of leads or other circuit components.
Verifying the existence and location of solder bumps and other features on the surfaces of electronic components is ever more difficult as the size of components decrease and the number of features increase. This is the case with ball grid array devices, which may have tens or hundreds of conductive solder bumps arranged in regular patterns. This is also the case with flip chips, whose surfaces include complicated "art work" and circuit traces, as well as solder bumps. Due to the small size and complexity of these devices, it is difficult to distinguish well-formed solder bumps on their surfaces from defective bumps and from other features, such as solder pads.
The prior art suggests a few approaches to finding solder bumps on circuit devices. The traditional approaches are primarily binary techniques based on two-dimensional "blob" analysis of the solder bump array images. A drawback of these approaches is that, due to their binary nature, they are highly susceptible to adverse lighting conditions or device-to-device variations. More significantly, they do not adequately distinguish properly formed solder bumps from imperfects bumps and solder pads.
Although not the subject of this invention, the art provides techniques for generating "height" images. Unlike conventional images (which are referred to herein as "intensity" images) containing pixels that represent the intensities (e.g., brightness or color) of corresponding portions of a scene, height images contain pixels that represent the heights of those corresponding portions.
An object of this invention is to provide improved methods for machine vision analysis and, particularly, improved methods for determining spatial characteristics of objects such as solder bumps.
More particularly, an object of the invention is to provide methods for interpreting height images to determine the spatial characteristics of objects shown in those images.
Yet another object of the invention is to provide machine vision methods for interpreting height images to provide information that permit three-dimensional objects, such as solder bumps, to be distinguished from two-dimensional objects, such as solder pads. A related object is to provide methods that provide information permitting well-formed three-dimensional objects to be distinguished from poorly formed three-dimensional objects.
Yet still another object of the invention is to provide such methods that can execute quickly, and without undue consumption of resources, on a wide range of machine vision analysis equipment.